The present invention relates to a tandem master cylinder for hydraulic brake systems for automotive vehicles. This tandem master cylinder is particularly useful in brake systems having a brake pressure control means, such as an anti-locking control means, and includes one or several valve elements, with at least one central valve housed within the push rod piston of the tandem master cylinder.
The ALFRED TEVES GMBH Brake Handbook, 8th edition, at pages 74 and 75, teaches a tandem master cylinder having two central valves.
German Offenlegungsschrift No. 3629564 describes a brake system which includes a slip control having a pedal-operated brake pressure generator, preferably supported by auxiliary force, to which the wheel brakes are connected through pressure conduits. This brake system further includes an hydraulic auxiliary pressure supply system having a hydraulic pump, a pressure equilibrium and pressure fluid reservoir and an auxiliary pressure control valve. This brake system also includes wheel sensors and electronic circuits for detecting the wheel rotation pattern and for generating electric brake pressure control signals capable of controlling electromagnetically actuated pressure fluid inlet valves and outlet valves located in the pressure fluid conduits to effect slip control.
The brake system in German Offenlegungsschrift No. 3629564 is characterized by a switch in the pressure conduit of the auxiliary force source which is actuated when a predetermined hydraulic pressure is reached. The switch provides an electric signal to the signal processing and logic circuit rendering the pump operative and inoperative during the test cycle and signals the driver, by an optical or acoustical indication, that the motor pump unit is ready to operate.
The lost travel of the central valve of the tandem master cylinder should be minimized. The lost travel, combined with the pedal transmission, results in unacceptably high lost travel of the brake pedal.
The two pistons of the tandem master cylinder should be arranged to be displaceable as a unit. Also, the two central valves of the tandem master cylinder should close simultaneously. Finally, the additive effects of lost motions and closing motions should be eliminated.